1. Field of the Invention:
The present invention relates to a GPS (global position system) receiver capable of shortening the time required initially to capture satellite transmissions.
2. Description of the Prior Art:
Transmissions (L.sub.1 wave) from GPS satellites are sent out at a frequency of 1575.42 MHz. Because the GPS satellites are not geostationary but orbiting satellites and because the GPS receiver is typically mounted on a moving body such as a vehicle for on-the-move signal reception, the Doppler-effect causes the receiving frequency of the transmission to fluctuate within a certain frequency range. Thus, to receive a desired satellite transmission requires changing the frequency within a predetermined satellite transmission search range in search for the target transmission, the search range covering the Doppler-shifted transmission variations.
FIG. 3 illustrates the construction of a typical prior art GPS receiver. In FIG. 3, reference numeral 1 is a multiplier for inverse diffusion; 2 is a multiplier for in-phase channel signal demodulation; 3 is a multiplier for orthogonal channel signal demodulation; 4 is a low pass filter that suppresses the harmonic component in the product from the multiplier 2 in order to obtain an in-phase channel signal I(t); 5 is a low pass filter that also suppresses the harmonic component in the product from the multiplier 3 so as to acquire an orthogonal channel signal Q(t); 6 and 7 are A/D converters that convert analog signals to digital format; 8 is a microcomputer for demodulation control; and 9 is a numerically controlled oscillator (NCO) controlled by the microcomputer 8.
The satellite transmission received with an antenna is converted to an intermediate frequency (IF) signal before being input to an input terminal of the multiplier 1 for inverse diffusion. Through another input terminal, the multiplier 1 admits PN code for identifying the target satellite. Maintaining the correlation between PN code and IF signal provides inverse diffusion and allows only the transmission from the target satellite to be extracted. The extracted signal is what is known as a PSK (phase shift keying) signal which is input to the multipliers 2 and 3.
The microcomputer 8 causes the NCO 9 successively to change the oscillation frequency thereof (reproduced carrier) in increments of a certain frequency range (usually a PLL capture range) throughout the entire satellite transmission search range established in advance. The multipliers 2 and 3 are supplied respectively with an in-phase component carrier and a 90.degree. phase shifted orthogonal component carrier, both generated by the NCO 9. The two multipliers demodulate the components. As a result, the in-phase channel signal I(t) containing the harmonic component is output from the output terminal of the multiplier 2, and the orthogonal channel signal Q(t) also containing the harmonic component is output from the output terminal of the multiplier 3. The two outputs pass through the low pass filters 4 and 5 wherein the harmonic component is removed from each channel signal. This leaves the in-phase channel signal I(t) and orthogonal channel signal Q(t) demodulated, the in-phase signal being the original base band signal.
The microcomputer 8 calculates a demodulation intensity I.sup.2 +Q.sup.2 using the in-phase channel signal I(t) and the orthogonal channel signal Q(t) coming from the A/D converters 6 and 7, the demodulation intensity indicating the status of satellite transmission reception at a given point in time. The frequency at which the demodulation intensity I.sup.2 +Q.sup.2 is maximized is detected as the receiving frequency f.sub.c of the satellite transmission. The microcomputer 8 sets the oscillation frequency (reproduced carrier) of the NCO 9 to the receiving frequency f.sub.c. Then a PLL loop is created on a software basis, the loop comprising the multipliers 2 and 3, the low pass filters 4 and 5, the A/D converters 6 and 7, the microcomputer 8. and the NCO 9. With the PLL loop established, the oscillation frequency of the NCO 9 (i.e., reproduced carrier) is locked in phase to the receiving frequency f.sub.c.
One disadvantage of the prior art GPS receiver is that it takes considerable time initially to capture the desired satellite transmission because, as described, the search frequency is consecutively changed in increments of the PLL capture range throughout the entire satellite transmission search range in search of the target transmission.